The present invention relates generally to medical laser imaging systems.
Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photographic film, a film processor for developing the film, and an image management subsystem for coordinating the operation of the laser imager and the film processor.
The image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the image management subsystem processes the image values to generate a sequence of digital laser drive (i.e., exposure) values, each of which is representative of one of a plurality of intensity levels (e.g., a grey scale) at a discrete pixel location in the image. The image processing electronics scales and maps the range of scanned image values to a range of laser drive values which will produce a useful, continuous tone photographic image. This mapping operation is necessitated by the nonlinear relationship between the input values and their visual representation, and by the nonlinear sensitometric response of the photographic film to different intensities of light. The image management subsystems of laser imagers commercially available from 3M of St. Paul, Minn. include a plurality of stored lookup tables which characterize the relationship between the image values and laser drive values. Each lookup table is configured for one of several types of film and specific image characteristics such as contrast and the maximum and/or minimum densities of the final image. The selected lookup table for a given image is accessed by the image management subsystem as a function of the image values to determine the associated laser drive values.
In addition to selecting a desired lookup table, users of the commercially available 3M laser imagers can adjust the contrast and density levels on images by manually actuating controls interfaced to the image management subsystem. However, these adjustments are made on a trial and error basis with test patterns, an inconvenient and inefficient procedure. Furthermore, the user is only able to exercise a limited degree of control over the overall imaging system transfer function by selecting one of the lookup tables and adjusting the contrast and density ranges implemented by these lookup tables. This approach also fails to account for drifts in the overall system transfer function that can be caused by factors such as the depletion of developer chemicals and lot-to-lot variations between the ideal and actual film sensitometric characteristics.
It is evident that there is a continuing need for improved laser imaging systems. In particular, there is a need for a laser imaging system capable of automatically adapting to variations in media sensitometric characteristics and media development parameters. The system should also be capable of accommodating a greater degree of user control over the overall imaging system transfer function. Any such imaging system must of course be able to accurately and efficiently implement these functions to be commercially viable.